[Illustration:  Experiment with a Block of Ice]

supported at each end by boxes BB, and a weight, W, is hung on a wire loop which passes around the ice as shown.  The pressure of the wire will then melt the ice and allow the wire to sink down through the ice as shown in Fig. 2.  The wire will continue to cut its way through the ice until it passes all the way through the piece, as shown in Fig. 3.  This experiment not only illustrates how ice melts under pressure, but also how it solidifies when the pressure is removed, for the block will still be left in one piece after the wire has passed through.

Another peculiar property of ice is its tendency to flow.  It may seem strange that ice should flow like water, but the glaciers of Switzerland and other countries are literally rivers of ice.  The snow which accumulates on the mountains in vast quantities is turned to ice as a result of the enormous pressure caused by its own weight, and flows through the natural channels it has made in the rock until it reaches the valley below.  In flowing through these channels it frequently passes around bends, and when two branches come together the bodies of ice unite the same as water would under the same conditions.  The rate of flow is often very slow; sometimes only one or two feet a day, but, no matter how slow the motion may be, the large body of ice has to bend in moving.

This property of ice is hard to illustrate with the substance itself, but may be clearly shown by sealing-wax, which resembles ice in this respect.  Any attempt to bend a piece of cold sealing-wax with the hands results in breaking it, but by placing it between books, as shown on page 65, or supporting it in some similar way, it will gradually change from the original shape A, and assume the shape shown at B.

** Return-Call Bell With One Wire [101]

[Illustration:  Wiring Diagram]

To use only one wire for a return call bell connect up as shown in the diagram, using a closed circuit or gravity battery, B. The current is flowing through both bells all the time, the same as the coils of a telegraph sounder, but is not strong enough to ring both connected in series.  Pressing either push button, P, makes a short circuit of that bell and rings the one at the other end of the line.  —­Contributed by Gordon T. Lane, Crafton, Pa.

** Circuit Breaker for Induction Coils [101]

Amateurs building induction coils are generally bothered by the vibrator contacts blackening, thus giving a high resistance contact, whenever there is any connection made at all.  This trouble may be done away with by departing from the old single-contact vibrator and using one with self-cleaning contacts as shown.  An old bell magnet is rewound full of No. 26 double cotton-covered wire and is mounted

[Illustration:  Interrupter for Induction Coil]

upon one end of a piece of thin sheet iron 1 in. by 5 in. as per sketch.  To the other end of the strip of iron is soldered a piece of brass 1/64 in. by 1/4, in. by 2 in., on each end of which has been soldered a patch of platinum foil 1/4 in. square.